The events of Sep. 11, 2001 forced recognition of an urgent need for more effective and stringent screening of airport baggage. The need for security expanded from the inspection of carry-on bags for knives and guns to the complete inspection of checked bags for a range of hazards with particular emphasis upon concealed explosives. The demonstrated willingness of terrorists to die in the pursuit of their aims meant that 100% passenger-to-bag matching, which could be put in place rapidly, was not sufficient to counter an attempt to conceal explosives in checked baggage and bring down an airliner. Successful screening for the presence of explosives presents numerous technological challenges, many of which are not met in present systems. For example, existing systems do not, and cannot, scan luggage rapidly and accurately for detecting the presence of explosives and hazardous materials. This is because data from a luggage scan may have a low signal-to-noise ratio, making it difficult to accurately scan the luggage. Also, since a luggage scan may require a comparison of the luggage content against a large number of reference substances, which require significant time, existing luggage scan may not be able to be performed rapidly.